Rotating disc memory storage devices are used in conjunction with digital computers to magnetically store digital information an non-volatile basis. A typical device of this type includes a spindle rotated by a drive motor mechanism, one or more recording discs attached to the spindle, either permanently or removably, for rotation therewith at a relatively precise constant speed, and an electromechanical read/write assembly for enabling information to be written onto and read from the disc recording surfaces. Each read/write assembly includes a number of transducers for reading and writing data magnetically from and to the individual disc recording surfaces, and a transducer motion translation mechanism, typically operated by a stepper motor, to effect radial motion of the transducers across the recording surface of each disc. The stepper motor is typically driven by conrol circuitry that receives electrical position signals in digital form for the associated computer, and converts this position information into mechanical motion of the transducer heads.
Each annular recording surface of a disc is usually arranged in the form of concentric circular tracks divided in the circumferential direction into track sectors, in order to enable access locations to be accurately specified by the associated computer for rapid and accurate information storage and retrieval.
Recent trends in the development of rotating disc memory storage devices have been toward reduction in the physical size of the system without sacrificing, and in many cases increasing, the storage capacity of each disc. These trends have been especially evident in disc drives designed for use in small business computers and personal computers. Such disc drives employ 51/4" or 31/2" fixed rigid discs, removable flexible (floppy) discs or a combination of both. Due to the relatively small surface area available for information storage on such discs, many efforts have been made to maximize the amount of information that can be accurately stored on such discs. These efforts have included a wide variety of specially designed recording techniques, read/write transducers with increasingly narrow heads (to reduce the trace width) and disc recording layers with improved magnetic recording properties and finer surface smoothness.
To obtain the maximum storage capacity for a fixed cost, it is desirable that each disc contain the maximum number of bits and tracks per inch. As the track density increases, however, it becomes increasingly difficult to repeatably precisely position the heads. Accordingly, it is clear that the upper track density limit in most state of the art systems is determined by how precisely the heads can be positioned over a selected track.
Positioning inaccuracy in state of the art disc drive devices is attributable to many factors. One such factor is stepper motor magnetic hysteresis. Hysteresis is the lag of magnetization behind magnetizing force as the magnetic condition of a ferromagnetic material is changed. The phenomenon can be explained with reference to FIG. 1 of this application. When a ferromagnetic sample that is initially demagnetized is subjected to a continuously increasing magnetizing force H, the relation between force H and flux density B is shown by the normal magnetization curve Oab of the Figure. This is the magnetization force.
The curve illustrates the phenomenon that occurs when a coil of a stepper motor is energized to cause movement of the stepper motor through one or more steps to change the position of a transducer head. The point a indicates the magnetic condition as the increasing magnetic intensity reaches H.sub.1. If magnetizing force H is increased to a maximum value H.sub.2, then decreased again to H.sub.1, the decreasing flux density does not follow the path of increase, but decreases at a rate less than that at which it rose. This lag in the change of flux density b behind the change of magnetizing force H is called "hysteresis." Even if the value of magnetizing force H is further reduced from H.sub.1 to zero, flux density b is not reduced to zero but to a value B.sub.r.
Applying this concept to the energized coils of a stepper motor, a coil, even when deenergized, retains some residual magnetization. Normally several but not all phases of a stepper motor are energized to select a given position for the motor. The residual magnetization of a coil which would not normally be energized in positioning a transducer over a track will result in mispositioning of the transducer relative to the desired track, and a consequent erroneous data read or data storage.